Coriolis Effect - practical considerations

[DannyS]

Agree Barry, they should quit while they are ahead, save their money and stay home.

Cheers
Danny

[Norm]

Mike, a rough rule of thumb is that Coriolis has 1/3 the horizontal effect of Spin Drift for LR high BC bullets.

In the southern Hemisphere you subtract Coriolis from spin drift for right hand twist barrels. Up in the Northern Hemisphere you add it to the Spin Drift so is a compounding problem.

So if your not taking Spin Drift into account then why worry about Coriolis?

If you are going to account for Coriolis then you should also account for Spin Drift. (Or you will be making your correction in the wrong direction)!

[DaveMc]

yes Norm -and at risk of being accused of team tactics again I will grab the results from Bryan Litzs book so you can all blame him instead of me.

284 180 gr VLD 2800fps has 8-9.8 inches of right spin drift at 1000 negated in part by about 2.5-2.8 inches of "apparent" left coriolis effect resulting in a net change in wind zero of around 5/8-3/4 of a minute to the right.

[fclass556]

Cant see what the concern is .. Coriolis is a serious consideration with a specific type of shooting where one cold bore shot is essential and thats not target shooting. You have the advantage 2 'practice' shots before the real shooting counts. For target shooters, it is another factor to consider more so on north / south facing ranges closer to the equator but realistically it's no different to winding on for wind when your sighters have been confirmed.

[bruce moulds]

556,
refer to what dave wrote about shooting in a fishtail.
this is what makes good shooters better, and what helped at raton.
when this subject came up prior to raton, there were people who belittled consideration of coriolus, but luckily some took note of it.
keep safe,
bruce.

[fclass556]

ok ...............

[IanP]

Interesting topic and really only gave it some thought when I was planning to shoot at Raton. I was concerned with the horizontal component which has already been discussed.

Interestingly Litz places more importance on the vertical component and gives an example in his book of a 155gr bullet with a velocity of 3000fps. He calculates on page 100, (AB For LRS book) the vertical component for a given azimuth (direction) and latitude.

If the rifle was zeroed at 1000 yards facing in one direction, (east) then fired in the opposite direction, (west) the vertical bullet drop would be an additional 6" attributed to Coriolis.

Thats enough of an effect for me to now add another one of Litz's formulas to my spreadsheets along with the direction of local ranges I shoot on. It looks like for long range shooting you could be wasting your first sighters by not initially correcting for the vertical component of this effect. His formula, (in his book) provides a gravity correction factor which you apply to calculated bullet drop of your ballistic program or table.

If you are correcting for the horizontal component, (I bet very few are) then you may be overlooking the larger effect of the vertical. Its made me rethink about this effect that up until now I have ignored. I'm betting the match rifle crowd are across this more so than the F-Class shooters!

Ian

PS Alan, it might be worth adding the range direction (azimuth) to your catalogue of range map data you are collecting?

[williada]

Coriolis
The Coriolis force can be a factor in explaining differences in rifle zero from your home range but it is insignificant once the rifle is re-zeroed at the new site. Of more importance is the degree target sits above the horizontal compared to your home range because this angle affects the compensating range of the rifle and can mimic a rifle out of tune. Of course the group dispersion will depend on whether your rifle is compensating neutrally, positively or negatively. Coaches also have to be aware that rifles using projectiles with different BC’s and barrels with different twist rates for instance, will have a different Magnus force which in turn (pardon the pun) will have a different influence on the projectile’s path and hence projectile dispersion. So alterations in windage and elevation will vary for different setups.
We know a watch that has stopped, can be right twice a day. Consider that a team zero can be right at a set distance and different at every other distance because of variances in the Magnus force if team members are using different gear. This is most relevant in a fishtail wind where it will impact on elevation. Most coaches can handle horizontal wind drift of different gear by generally learning the force and angle of wind. Other technicals can become self evident by team practice at different distances if you are trying to measure what you should be looking for. Sometimes it is not that much, but enough to drop that centre count. Just food for thought.

[fclass556]

Coriolis
The Coriolis force can be a factor in explaining differences in rifle zero from your home range but it is insignificant once the rifle is re-zeroed at the new site. Of more importance is the degree target sits above the horizontal compared to your home range because this angle affects the compensating range of the rifle and can mimic a rifle out of tune. Of course the group dispersion will depend on whether your rifle is compensating neutrally, positively or negatively. Coaches also have to be aware that rifles using projectiles with different BC’s and barrels with different twist rates for instance, will have a different Magnus force which in turn (pardon the pun) will have a different influence on the projectile’s path and hence projectile dispersion. So alterations in windage and elevation will vary for different setups.
We know a watch that has stopped, can be right twice a day. Consider that a team zero can be right at a set distance and different at every other distance because of variances in the Magnus force if team members are using different gear. This is most relevant in a fishtail wind where it will impact on elevation. Most coaches can handle horizontal wind drift of different gear by generally learning the force and angle of wind. Other technicals can become self evident by team practice at different distances if you are trying to measure what you should be looking for. Sometimes it is not that much, but enough to drop that centre count. Just food for thought.

Know your rifles capabilities and zero .. 2 sighters to centre .. Coriolis gone !! .. For those that say there's more to it than that you're just making it more complicated for yourselves. Thanks you williada Your explanation was more eloquent then mine ..

[DaveMc]

AHHH - Magnus Force the most often misquoted force in rifle shooting.

I take it you are talking about the 10:4 slope generally associated with increasing/decreasing side winds and fishtails?? Often referred to as Aerodynamic Jump????

A challenge for those with a bit of a technical mind. Although McCoy and other earlier ballisticians correctly identified an effect and their formulas roughly approximate the flight path under different conditions it is now believed this is a gyroscopic force and Magnus acts in the other direction (effectively slowing it over longer ranges such that a 15-20 degree slope at short range will flatten to a 5 degree (or less effectively flat line) at long range.

Harold Vaughn first highlighted this issue but to demonstrate I think the best two pictures for this are located here:
http://en.wikipedia.org/wiki/Magnus_effect
Notice the wind from the left in diagram with right spin bullet - if the increasing wind strength in this case would cause a 8-2 slope (not 10-4)
and here:
http://en.wikipedia.org/wiki/Gyroscope
Gyroscopic forces are in the correct direction for this. Also as the projectile turns into the wind it is thought there is a larger initial gyroscopic moment causing a steeper slope at the start. then the magnus forces act against this gradually reducing the effect.

Although Wikipedia can have errors these diagrams are correct and simply show the direction of the forces.

Benchrest shooters know more about this and see it more dramatically at shorter ranges.


Those that want to say "don't worry about it get on and shoot. It doesn't matter" Fine - I understand your thoughts - we have all heard it before. This is for those that want to know more detail and understand what is going on. Leave your negativity at home and turn over to another thread if you are not interested!

And as far as wind zero being correct for one range and not another you are totally correct but at least with a little understanding you can add or take away a click or two as you move through the ranges and keep it approximately in the middle.

Those that are happy shooting with wind zeros out by half to 1 minute - all the best of luck to you (and I do appreciate that some do this far better than I do.)

[Norm]

A simple question.

Coriolis and Spin drift aside.

Considering wind direction on elevation only. (For a flat surface).

How come ballistic programs do not show any variation in their solution output for elevation, when a wind value is changed from full value 3 O'clock to full value 9 O'clock.

If these effects were true, well known and significant, why do the programs not take them into account?

[AlanF]

A simple question.

Coriolis and Spin drift aside.

Considering wind direction on elevation only. (For a flat surface).

How come ballistic programs do not show any variation in their solution output for elevation, when a wind value is changed from full value 3 O'clock to full value 9 O'clock.

If these effects were true, well known and significant, why do the programs not take them into account?

Probably because there aren't widely accepted theories on how to calculate them. I can tell you that Canberra range has a lot of the "right wind raises" effect, but it has a substantial side slope. But when the wind "flops over" just about anywhere it effects elevation.

[DaveMc]

Norm - most ballistics programs are 3 DOF at best. A true 6 Degrees of Freedom model will have more detail and should include these factors. BUT there are very few projectiles we actually have all the necessary data for.

Page 267-270 of Robert McCoys Modern Exterior Ballistics has some equations.

We need Projectile axial moment of inertia and other bullet measurements which are not easy to obtain.

[Norm]

Dave,
Do you have any idea of what magnitude of effect we are talking about?
At 1000 yards would it be more than or less than 1 MOA for a 7mm bullet in a full value 3 O'clock right angle side wind?

[DaveMc]

Norm - The McCoy calculations showed a 23 degree slope for a 308 168 grain out of a 12 twist barrel at 100 yards (This is supported by many benchrest observations and I have a couple of pages of calculations I did here and might get them to Alan to post). I was in the middle of doing some 105 grain scenar measurements when I got sidetracked by the US trip and realisation that the formulas weren't giving us the exact relationship to what was happening in the field. Some of our measurements and calculations are more like 5-6 degrees at 1000 yards and up to 20 degrees at short range. We sent back all our plots from Raton to Peter Smith between the Nationals and worlds as AJ was quite obvious in the plots there with the switching tailwind. We used this successfully to save a few points in the worlds.

As Alan has noted there are many other factors that can affect this and we have seen reverse slope on some ranges as well as steeper and shallower.